ARM: mxs_defconfig: Enable RTC driver

Let RTC driver driver be built by default.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>

arch/arm/configs/mxs_defconfig

@@ -114,6 +114,7 @@ CONFIG_MMC=y
 CONFIG_MMC_MXS=y
 CONFIG_RTC_CLASS=y
 CONFIG_RTC_DRV_DS1307=m
+CONFIG_RTC_DRV_STMP=y
 CONFIG_DMADEVICES=y
 CONFIG_MXS_DMA=y
 CONFIG_EXT3_FS=y